Elastic elastomers

Elastomers. Elastomers is a generic name for polymers that exhibit rubberlike elasticity. Elastomers are soft yet sufficiently elastic that they can be stretched several hundred percent under tension. When the stretching force is removed, they retract rapidly and recover their original dimensions. 

J.-M. Charrier, Eolymeric Materials andErocessing Elastics, Elastomers and Composites, Hanser PubHshers, New York, 1991, Chapt. 5. 

Synthetic rubber (elastomers) are high molecular weight polymers with long flexible chains and weak intermolecular forces. They have low crystallinity (highly amorphous) in the unstressed state, segmental mobility, and high reversible elasticity. Elastomers are usually cross-linked to impart strength. 

Elastomer (DDR Elaste), Elastomer (GummiAVeichgummi leicht vemetzte Synthesekautschuke)... 

Before we proceed, a word of caution is necessary here in terms of our use of the parameter In fracture mechanics, has the meaning of an energy per unit area necessary for a crack to advance. For elastic elastomers, is a unique function of the velocity of the advancing crack and can be determined with a fracture mechanics test such as the JKR test [24]. 

As the demand for rubber increased so did the chemical industry s efforts to prepare a synthetic sub stitute One of the first elastomers (a synthetic poly mer that possesses elasticity) to find a commercial niche was neoprene discovered by chemists at Du Pont in 1931 Neoprene is produced by free radical polymerization of 2 chloro 1 3 butadiene and has the greatest variety of applications of any elastomer Some uses include electrical insulation conveyer belts hoses and weather balloons... 

Edman degradation (Section 27 13) Method for determining the N terminal amino acid of a peptide or protein It in volves treating the material with phenyl isothiocyanate (CgH5N=C=S) cleaving with acid and then identifying the phenylthiohydantoin (PTH derivative) produced Elastomer (Section 10 11) A synthetic polymer that possesses elasticity... 

Polyolefins. In these thermoplastic elastomers the hard component is a crystalline polyolefin, such as polyethylene or polypropylene, and the soft portion is composed of ethylene-propylene rubber. Attractive forces between the rubber and resin phases serve as labile cross-links. Some contain a chemically cross-linked rubber phase that imparts a higher degree of elasticity. 

Polymers with the mechanical and chemical properties we have discussed in this section are called elastomers. In the next couple of sections we shall examine the thermodynamic basis for elasticity and then apply these ideas to cross-linked polymer networks. 

It is necessary to establish some conventions concerning signs before proceeding further. When the applied force is a tensile force and the distortion is one of stretching, F, dL, and dw are all defined to be positive quantities. Thus dw is positive when elastic work is done on the system. The work done by the sample when the elastomer snaps back to its original size is a negative quantity. 

Since entropy plays the determining role in the elasticity of an ideal elastomer, let us review a couple of ideas about this important thermodynamic variable ... 

A great many liquids have entropies of vaporization at the normal boiling point in the vicinity of this value (see benzene above), a generalization known as Trouton s rule. Our interest is clearly not in evaporation, but in the elongation of elastomers. In the next section we shall apply Eq. (3.21) to the stretching process for a statistical—and therefore molecular—picture of elasticity. 

At still longer times a more or less pronounced plateau is encountered. The value of the plateau modulus is on the order of 10 N m", comparable to the effect predicted for cross-linked elastomers in Sec. 3.4. This region is called the rubbery plateau and the sample appears elastic when observed in this time frame. 

Methylenebis(4-phenyl isocyanate). This compound is also known as methyl diisocyanate [101-68-8] (MDI) and is produced by the condensation of aniline and formaldehyde with subsequent phosgenation. Its principal end use is rigid urethane foams other end uses include elastic fibers and elastomers. Total formaldehyde use is 5% of production (115). 

At strains over 300% the stress occurs mostiy in the amorphous regions up to the point where the sample breaks. AH of the grades exhibit permanent set, and the curves of grades with a Shore Hardness of 55 and higher exhibit a yield point. This means that parts have to be designed for low strains to stay within the area of elastic recovery. Special grades of elastomer are available to provide hydrolysis resistance (194), improved heat aging (195), and improved uv-stabihty (196). 

Desirable properties of elastomers include elasticity, abrasion resistance, tensile strength, elongation, modulus, and processibiUty. These properties are related to and dependent on the average molecular weight and mol wt distribution, polymer macro- and microstmcture, branching, gel (cross-linking), and... 

The melt temperature of a polyurethane is important for processibiUty. Melting should occur well below the decomposition temperature. Below the glass-transition temperature the molecular motion is frozen, and the material is only able to undergo small-scale elastic deformations. For amorphous polyurethane elastomers, the T of the soft segment is ca —50 to —60 " C, whereas for the amorphous hard segment, T is in the 20—100°C range. The T and T of the mote common macrodiols used in the manufacture of TPU are Hsted in Table 2. 

Elastomer Impression Materials. Dentistry requires impression materials that are easily handled and accurately register or reproduce the dimensions, surface details, and interrelationship of hard and soft oral tissues. Elexible, elastomeric materials are especially needed to register intraoral tooth stmctures that have undercuts. The flexibility of these elastomers allows their facile removal from undercut areas while their elasticity restores them to their original shape and size. 

Polysulfide Impression Materials. In 1953 the first nonaqueous, elastic dental impression material based on the room-temperature conversion of a Hquid polymer, a polyfunctional mercaptan (polysulfide), to a strong, tough, dimensionally accurate elastomer, was introduced. The conversion of the Hquid polymer to an elastic soHd has been achieved in most products by lead peroxide [1309-60-0]. Significant improvements in strength, toughness, and especiaHy dimensional stabiHty of the set polysulfide elastomers over the aqueous elastic impression materials made these materials popular. 

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